Steel Roof Grounding

[Industrial Toys]

Something I have pondered before. Whether to ground a steel roof for lightning, whether you can do this effectively and how?

I am replacing my house roof with steel sheets, formed to look like shakes. We have lightning rods, but they wouldn't seem to make too much sense if the roof is steel.

There must be a great many steel, ungrounded roofs that have had lightning hits. Does it take a hit and then flash over to the closest grounded conductor, say, inside wiring? This has always worried me on our barn, which has no lightning rods, but so far this has never appeared to have happened.

Any thoughts or experience?

Thanks

 

[Rock knocker]

We have lightning rods, but they wouldn't seem to make too much sense if the roof is steel.

Why not?

A manufacturers response:
Questions about metal roofs

 

[Industrial Toys]

Well, I mean if the grounding system is still in place. The rods would only be inches above the steel roof. And besides, I have a steel plate over my chimney flues. That is higher and is grounded.

 

[teejk]

A few years ago I was in my metal shop building during a thunderstorm actual actually saw a blue lighting flash follow my radio antenna wire into the building (almost wet myself at the time and wondered what might have happened if I was closer). I guess there was just enough connectivity to send it back to the house ground rods but when I think about it I wonder why since it is a typical pole barn construction and there is very little metal on metal contact that would allow lightening to go to ground. Interesting question though. More interesting is the recent NEC mandate for "single point grounding".

 

[Industrial Toys]

I studied single point ground extensively a few decades ago when using polyphaser products in my repeater shack. There is grounding for life safety and to prevent a place from burning down, but a real single point ground system will save your electronic goodies. It is hard enough to implement in a small radio shack and almost impossible in the average house.

 

[Rustyiron]

I think your existing lightning protection would take the "hit" first, no matter the roofing material.

 

[CalG]

You may have the concept of 'lightning protection" somewhat misconstrued.

Lightening can carry sufficient current to burn away almost any reasonable conductor. (wire). The goal of "lightening rods" is to dissipate the electrical potential in the ground around a building INTO the atmosphere, and so lower the voltage differential between earth and sky.
The key component of such systems is the pointed terminal at the high point.

"grounding" a metal roof is a fools errand, though bonding all metal structure to a good earth "ground plane" is always a good idea, especially in acid soils to limit galvanic corrosion.

If you think that a direct hit can be carried away by a few strands of double ought copper wire, and everything below will be safe in isolation, well, that's not going to happen.

I live on a hill, I'm not sure just why, but in the 30 years we have lived here, we have had several significant strikes to surrounding tree.
two lost well pumps, and three trees blown to smitherines. dead computers, burned through phonelines, yes, burned through!, and two lost VFDs in the machine shop, one a 25 HP device. And the house has a metal roof. It's the trees with their connecting root structure that provides the neutralizing path to ground.

If you have no trees near by, an 8 gauge wire connected to an always damp electrical ground rod (separate from your service ground) and attached to a series of wicks mounted on the ridge of your roof is the way to go.

 

[Industrial Toys]

I beg to disagree. A good sturdy conductor can carry a lot of lightning current. One thing though. A given amount of ground can only suck up so much of a charge.In my radio shack grounding project, I believe there may have been as many as twelve ground rods dispersed far and wide.

And actually, apparently lightning is more like RF travellling on the surface of a conductor, making flat copper strips one of the better grounding conductors. Which does kind of make me wonder, how lightning may negotiate the flat steel roofing material.

 

[PILOON]

4 of us were sitting in a living room during a storm in a chalet surrounded by tall trees while lighting and thunder roared.
Suddenly a loud crash and a bolt struck and the hanging chandelier exploded in front of our eyes with a blue flash.
Other than blowing every light bulb in the house (and staining our short) there was no other ill effects.

In all probability the bolt hit the hydro power line causing an over voltage and not a direct hit.

Lightening will hit the highest best conductor which generally be a nearby tree or the power line.(or you on the golf course jolding a metal umbrella).

 

[CalG]

I beg to disagree. A good sturdy conductor can carry a lot of lightning current. One thing though. A given amount of ground can only suck up so much of a charge.In my radio shack grounding project, I believe there may have been as many as twelve ground rods dispersed far and wide.

And actually, apparently lightning is more like RF travellling on the surface of a conductor, making flat copper strips one of the better grounding conductors. Which does kind of make me wonder, how lightning may negotiate the flat steel roofing material.

Disagree if you wish.

Here is an "off the cuff" comment extracted from the web.

quote:
From articles in Windpower Engineering & Development, we learn that lightning bolts carry from 5 kA to 200 kA and voltages vary from 40 kV to 120 kV. So if we take some averages, say, 100 kA and 100 kV, this bolt would carry this much power,
[end quote]

What size wire can carry 5000 Amps to 200,000 Amps? Super conductor wire?